This invention relates to a system of testing magnetic recording tape. Magnetic recording tape is generally subject to defective portions which are referred to as "drop outs", and if any such defective portions are present, it becomes impossible to accurately write or read information thereon or therefrom. Accordingly, in the case of magnetic tape for digital recording to be used with computers or similar equipment, tests are made for checking the absence of such defective portions, so as to guarantee each reel of magnetic tape for complete absence of defective portions or for the minimum number of possible defective portions if not zero.
There are two types of defective portions in magnetic tape depending on the causes; namely, one type caused by defects in the magnetic layer of the tape itself, such as uneven application of the magnetic layer and damages to the magnetic layer, and another type caused by incomplete contact between magnetic heads and the magnetic recording tape due to dust particles or other foreign matter deposited on the tape, so as to hamper correct writing and reading. The former type is referred to as "permanent defective portions", while the latter type is referred to as "temporary defective portions".
If any defective portions are found on the magnetic recording tape, in order to determine whether such defective portions are of permanent type or temporary type, it has been usual practice to repeat writing and reading a plurality of times at each defective portion, so that if the defect is eliminated during the repetition, the defective portion is assumed to be of temporary nature, while if the defect remains until the end of the repetition it is assumed to be of permanent nature.
FIG. 1 illustrates a circuit diagram of conventional equipment for testing the defective portions of the magnetic recording tape. In the figure, 2 and 3 are a capstan and a pinch roller, respectively, for feeding the magnetic recording tape 8 (to be referred to simply as "tape", hereinafter) in a forward direction (the direction as shown by the arrow in FIG. 1) between the reels 20 and 21. As the tape is fed in the aforesaid manner, predetermined signals from a recording circuit 4 are applied to a recording-reproducing head 1 (to be referred to simply as "RR head", hereinafter), so as to record the signal on the tape 8 and read out immediately afterwards. The signal thus read out is amplified and shaped by a read circuit 5, so as to properly reproduce the signal recorded. If the tape has any defective portions, the signal reproduced is interrupted at the defective portion, so that a defective-portion-detecting circuit 6 generates a defect signal at the output terminal 25. This defect signal acts to terminate the forward tape feeding and causes the tape 8 to be fed in the reverse direction (the direction contrary to that of the arrow) by bringing a reverse capstan 2' and a reverse pinch roller 3' into contact with the tape 8, so as to bring the defective portion to the left of the RR head 1 in FIG. 1. Then, the tape 8 is fed again in the forward direction for rechecking the defective portion.
Thus, that short portion of the tape which may include a defect is repeatedly fed forward and reverse for testing for the presence of the defective portion. After a predetermined number of times repeating the aforesaid checking through the forward and reverse tape travel, if the defect signal is still present that defective portion is regarded as permanent, while if the defect signal is removed before completing the predetermined number of the aforesaid repetitions, that defective portion is regarded as temporary.
This conventional method, however, has the following shortcomings.
(1) In order to repeat the forward and reverse tape feeding in a short period of time, quick feed and quick stop of the tape is necessary. As a result, a complicated tape feed control mechanism is required, making the test device costly.
(2) Due to the same reason, the tape is exposed to the danger of damage during the aforesaid repetition.
(3) Since a finite time is necessary for switching from feed to stop and vice versa, the total time necessary for the test is long.
(4) The time for reverse feed is wasteful as far as the actual test is concerned.